Many optical apparatuses use various transmissive optical elements. For example, an imaging optical system of a digital camera uses lenses as the transmissive optical elements to form an object image on an image-pickup element.
However, optical glass and optical plastic generally used as materials of the transmissive optical elements have a large refractive index, thereby having a large reflectance. Therefore, use of a plurality of such transmissive optical elements increases the amount of reflecting light, thus decreasing the amount of transmitting light.
In order to suppress the reflection by the transmissive optical element, the optical element is often provided with a reflection suppressing function. Some methods have been known as a method of providing an optical element with the reflection suppressing function.
The most common method is to form a reflection suppressing (antireflection) film on a surface of the transmissive optical element. More specifically, a thin film is formed on the surface of the transmissive optical element by a thin-film formation technology such as vapor deposition or sputtering, so as to reduce the reflectance by using optical interference.
Another method is to use a structure finer than an entering light wavelength (also called a use wavelength). As the most famous structure, a “moth-eye” structure is available. The moth-eye enables a very low reflectance due to its unique minute structure.
In the structure finer than the entering light wavelength, light is not affected by the structure, but shows a property as if it passed through a uniform medium. Such a structure shows a refractive index in accordance with a volume ratio of the materials constituting thereof. Thereby, a structure with a low refractive index can be achieved, which cannot be achieved by a normal material, thus favorably suppressing the reflection.
As the reflection suppressing method using the minute structure, a particle-dispersed film is applied in which minute particles with a diameter smaller than the entering light wavelength are dispersed (Japanese Patent No. 3135944), and a minute periodic structure is formed by a minute-pattern processing technique (Japanese Patent Laid-Open No. 50 (1975)-70040).
However, a complicated process is required for forming the structure finer than the entering light wavelength, and the degree of freedom of design is low because of the limitation of materials constituting thereof. Thus, high reflection suppressing performance can be achieved with such a minute structure only for a transmissive optical element in limited conditions.
The present invention provides an optical element having optical performance such as good reflection suppressing performance and having a high degree of freedom of design.